1. Field of the Invention
The invention relates to a biodegradable material, more particularly to a biodegradable plastic material and a manufacturing method thereof.
2. Description of the Related Art
Nowadays, chemically synthetic plastic products, which are mainly made of petrochemical raw material, are widely used, for example in daily needs for food, clothing, living, or transportation. Nevertheless, with the rapid industrial development in developing countries, the need for various petrochemical raw materials has increased, arising in an increase of petroleum price together with their production and manufacturing costs. Therefore, the related industrial sectors are always in search of highly efficient and low costing manufacturing processes of plastic products.
Besides, ordinary chemically synthetic plastic materials are inherently stable and do not degrade easily under the natural environment, thereby resulting in serious environmental pollution. Developing biologically compatible and degradable plastic materials, such as plastic-like materials that are able to be decomposed into CO2 and H2O by microbes under the natural landfill environment, has therefore become a global trend. Three types of such plastics have been developed: photodegradable plastics, biodegradable plastics, and the disintegradable plastics.
The photodegradable plastics are degradable by UV solar energy, which induces chain reactions promoted by photo-sensitive agents in the plastic and breaks chemical bonding between the polymers of the plastic. However, most waste plastic products are either buried or sunk under water, which prevents any efficient degradation by solar energy.
Besides, the biodegradable plastics use natural polymers or aliphatic polyesters as a matrix, and a material such as starch, fiber, protein, or polylactic acid (PLA). Nevertheless, the high prices and poor strengths of such polymers restrict their applications.
The disintegradable plastics are made by mixing conventional plastic materials with starch or biologically fermented substances which are decomposable by microbes ubiquitous in the natural environment. The plastics are then disintegrated, thereby reducing the overall garbage together with the amount of petroleum raw material and the cost of manufacturing process.
Ethylene vinyl acetate (abbreviated as EVA) copolymer is a main raw material commonly used in shoe material, and is derived from the by products of the petroleum industry. Like other plastic materials, EVA materials face the problems of increasing cost of petrochemical oil price, non-degradability and environmental pollution at the end of its life cycle.
A manufacturing method of biodegradable plastic material is disclosed in ROC application no. 93118611 Taiwan patent no. 1248957). According to this method, starch is first dehydrated between 160° C. and 170° C. and ground into granules each having a size smaller than 10 μm, and then mixed and stirred with a biodegradable plastic raw material and a conventional chemically synthetic plastic raw material in order to form the biodegradable plastics.
According to another method disclosed in ROC application no. 88105965 Taiwan patent no. 400358), a biodegradable EVA foam is made using EVA as a matrix which is mixed with biodegradable components, such as cereal shell powder, wood saw dust, or starch, such as unprocessed corn starch.
ROC application Nos. 79105530, 79105777 and 79105778 Taiwan patent nos. 191226, 192556 and 191227, respectively) suggest to heat starch and plastic raw materials for a sufficient time period in an enclosed chamber and under adequate temperature and pressure so as to decompose the starch and form a homogenous melt for manufacture of plastic products.
Nevertheless, the biodegradable materials and the plastic raw materials are poorly compatible and provide weak resultant bonding strength therebetween. Therefore, the proportion of the biodegradable material in admixture with the plastic material is low. If the proportion of the biodegradable material is increased, the structural strength of the product can decrease, thereby limiting the application of the resultant product. For example, such materials are not suitable for use in plastic articles that require durability, for example, shoe soles prone to frequent wearing.
Besides, although the methods disclosed in ROC application nos. 79105530, 79105777 and 79105778 (Taiwan patent nos. 191226, 192556 and 191227, respectively) are able to increase the proportion of starch in the overall manufacturing materials, increasing temperature and pressure during the manufacturing process inherently increases the cost of energy supply and lengthens manufacturing time, which result in poor manufacturing efficiency and higher manufacturing costs.
Moreover, the foregoing disclosures of biodegradable materials are not suitable for use as shoe materials as their structural strengths do not comply with the standards for shoes.